The present invention relates to a method and a device for measuring an alternating electric quantity. In this case, an alternating electric quantity is understood to be an electric quantity which varies with time and whose frequency spectrum lies above a predetermined frequency. In particular, the alternating quantity can be an alternating electric current, an alternating electric voltage, or even an alternating electric field.
Optical measuring methods and measuring devices for measuring electric quantities such as current, voltage, or field are known, in which the change in polarization of polarized measuring light in a sensor device as a function of the electric quantity is evaluated. In this case, the magneto-optical Faraday effect is used for measuring an electric quantity; for measuring electric voltages and fields, on the other hand, the electro-optical Pockels effect is used.
The Faraday effect is understood to be the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The angle of rotation is, in this case, proportional to the path integral over the magnetic field along the path traced by the light, with the Verdet constants as constants of proportionality. The Verdet constant depends on the material in which the light runs and on the wavelength of the light. To measure an electric current in a current conductor using the Faraday effect, a Faraday element, which consists of an optically transparent material, such as glass, is arranged as a sensor device in the vicinity of the current conductor. Linearly polarized light is sent through the Faraday element. The magnetic field generated by the electric current effects a rotation of the plane of polarization of the light in the Faraday element around an angle of rotation. The angle of rotation can be evaluated by an evaluation unit as a measure of the intensity of the magnetic field and thus of the strength of the electric current. In general, the Faraday element surrounds the current conductor, so that the polarized light circulates around the current conductor in a quasi-closed path. In this case, the amount of the angle of rotation of the polarization is, to a good approximation, directly proportional to the amplitude of the measured current.
WO 91/01501 discloses an optical measuring device for measuring an electric current, having a Faraday element which is designed as part of an optical monomode fiber. The Faraday element surrounds the current conductor in the form of a measuring winding.
EP-B-0 088 419 discloses an optical measuring device for measuring a current, in which the Faraday element is designed as a solid glass ring around the current conductor.
The electro-optical Pockels effect is understood to be the change in the polarization of polarized measuring light in a material exhibiting the Pockels effect as a result of a linear birefringence that is induced in the material. On the basis of the electro-optical coefficients, the pockels effect is essentially linearly dependent on an electric field penetrating the material. To measure an electric field, a Pockels element made of a material showing the Pockels effect is arranged in the electric field as a sensor device. To measure an electric voltage, the voltage to be measured is applied to two electrodes assigned to the Pockels element and the corresponding, adjacent, electric field is measured. Polarized measuring light is transmitted through the Pockels element, and the change in polarization of the polarized measuring light as a function of the voltage to be measured or of the field to be measured is evaluated with the aid of a polarization analyzer.
In order to measure a current using a Faraday element and a voltage using a Pockels element, a polarizing beam splitter or a simple beam splitter with two polarizers arranged downstream can be provided as a polarization analyzer (EP-B-0 088 419 and DE-C-34 04 608). The polarizing beam splitter may comprise a Wollaston prism. The polarized measuring light which has passed through the sensor device is split in the analyzer into two linearly polarized, partial light beams A and B with planes of polarization which are different and generally directed at right angles to each other.
Each of these two partial light signals A and B is converted in each case by a photoelectric transducer into electric signals PA and PB.
In optical measuring methods and measuring devices, the polarization change of polarized measuring light in a sensor element, which is influenced by the measured quantity, is used as a measuring effect. A problem in such devices is presented by the sensitivities of the optical materials of the sensor element and of the optical transmission paths to temperature changes or to mechanical stresses which, for example, are brought about by bending or vibrations. In particular, in the case of changes in the system temperature, the temperature sensitivity leads to an undesired change in the operating point and in the measuring sensitivity (response to temperature changes).
Various temperature compensation methods are already known for compensating temperature influences.
A temperature compensation method for a magneto-optical measuring device for measuring alternating currents is known from Proc. Conf Opt. Fiber Sensors OFS 1988, New Orleans, pages 288 to 291, and the associated U.S. Pat. No. 4,755,665. In this method the two electric signals PA and PB belonging to the partial light signals A and B of the measuring light are each decomposed in a filter into their DC components, PA(DC) or PB(DC), and their AC components, PA (AC) or PB (AC). From the AC component PA(AC) or PB(AC) and the DC component PA(DC) or PB(DC), for each signal PA and PB, in order to compensate for different intensity fluctuations (attenuations) in the two transmission paths for the light signals A and B, the quotient QA=PA(AC)/PA(DC) or QB=PB(AC)/PB(DC) is formed from its AC component PA(AC) or PB(AC) and its DC component PB(DC) or PA(DC). From each of these two quotients QA and QB, a mean time value MW(QA) and MW(QB) is formed, and from these two mean values MW(QA) and MW(QB) a quotient Q=MW(QA)/MW(QB) is finally formed. Within the framework of an iteration process, a correction factor K is obtained for the determined quotient Q by means of comparison with calibrated values stored in a value table (look-up table). The value Q*K, corrected by this correction factor K, is used as a temperature-compensated measured value for an electric alternating current to be measured. Using this method, the temperature sensitivity can be reduced to approximately 1/50.
EP-A-0 557 090 discloses a temperature-compensation method for an optical measuring device for measuring magnetic alternating fields. This method uses the Faraday effect and is therefore also suitable for measuring electric alternating currents. In the case of this known method, once again the linearly polarized measuring light, after passing through the Faraday element, is split in an analyzer into two differently, linearly polarized partial light signals A and B. In order, to normalize the intensity, for each of the two associated electric signals PA and PB, the quotient QA=PA(AC)/PA(DC) or QB=PB(AC)/PB(DC) is formed separately from its associated AC component PA(AC) or PB(AC) and its associated DC component PA(DC) or PB(DC). In a computing unit, a measured signal M=1/((.alpha./QA)-(.beta./QB)) is now formed from the two quotients QA and QB, in which the real constants .alpha. and .beta. fulfill the relationship .alpha.+.beta.=1. This measured signal M is described as largely independent of changes in the Verdet constants and the circular birefringence in the Faraday element caused by temperature changes. Nothing is said about compensation of the temperature-induced linear birefringence.
EP-A-0 486 226 discloses a corresponding temperature compensation method for an optical measuring device for measuring an electric alternating voltage. An optical series circuit is connected optically between a light source and an evaluation unit. The optical series circuit consists of a polarizer, .lambda./4-plate, a Pockels element, and a polarized beam splitter. The beam splitter is used as an analyzer. The order of .lambda./4-plate and Pockels element in the optical series circuit can, however, also be exchanged. The measuring light from the light source is linearly polarized in the polarizer and, after passing through the Pockels element, is split in the analyzer into two partial light signals A and B having different planes of polarization. Each of these partial light signals A and B is converted into a corresponding electric intensity signal PA or PB. Thereupon, for normalizing the intensity for each of these two electric intensity signals PA and PB, the quotient QA=PA(AC)/PA(DC) or QB=PB(AC)/PB(DC) is formed from its associated AC signal component PA(AC) or PB(AC) and its associated DC signal component PA(DC) or PB(DC). From the two intensity-normalized quotients QA and QB, a measured signal M=1/((.alpha./QA)-(.beta./QB)) is now formed with the real constants .alpha. and .beta. in a computing unit. By matching these constants .alpha. and .beta., the measured signal M becomes largely independent of linear birefringence in the .lambda./4-plate caused by temperature changes.